1. Field of the Invention
The present invention pertains to radio frequency identification devices and, more particularly, to a radio frequency tag configured to operate in multiple modes for increased range and capability with more efficient use of power.
2. Description of the Related Art
Radio frequency identification (RFID) devices are utilized in systems that include an interrogator for transmitting interrogation signals and receiving responsive signals therefrom, and transponders that receive the interrogation signals and generate the responsive signals. In most systems, the transponders, known as “tags,” are either powered by a battery for active transmission or are powered by the interrogation signal for passive backscattering of a modulated signal. Semi-active tags typically operate in a passive mode but they have the ability to utilize stored energy to modulate the received signal.
The beam-powered RF tag is often referred to as a passive device because it derives the energy needed for its operation from the interrogation signal beamed at it. The tag rectifies the field and changes the reflective characteristics of the tag itself, creating a change in reflectivity that is seen at the interrogator. A battery-powered semi-passive RFID tag operates in a similar fashion, modulating battery-powered semi-passive RFID tag operates in a similar fashion, modulating its RF cross-section in order to reflect a delta to the interrogator to develop a communication link. Here, the battery is the source of the tag's operational power. Finally, in the active RF tag, a transmitter is used to generate its own radio frequency energy powered by the battery.
Referring to FIG. 1, a typical RF tag system will include an interrogator 12 and one or more RF tags 14. The range of communication for such tags 14 varies according to the transmission power of the interrogator 12 and the type of RF tag 14 used in the system 10. Battery-powered tags operating at 2,450 MHz have traditionally been limited to less than 10 meters in range. However, devices with sufficient power can reach up to 200 meters in range, depending on the frequency and environmental characteristics.
Conventional continuous wave backscatter RF tag systems utilizing passive RF tags require adequate power from the interrogation signal 20 to power the internal circuitry in the RF tag 14 that is used to amplitude-modulate the response signal 22 back to the interrogator 12. While this is successful for tags that are located in close proximity to the interrogator 12, for example less than 3 meters, this may be insufficient range for some applications, for example, which require greater than 100 meters.
Because passive RF tags 14 require the use of power directly from the interrogation signal 20, obtaining sufficient power to operate the tags 14 with enough sophistication to modify memory, monitor inputs, and control outputs dictates that the tags 14 be in close proximity (typically less than 1 meter) to the interrogator 12. It would be desirable to have a tag that can be detected at further distances so it can be located and identified, whereupon one could move closer to the tag in order to activate more complex functions in the tag. Conventional tags are either very close-range tags with sophisticated circuitry that requires substantial power or longer range tags that are very simple tags capable of only indicating their presence in an RF field.